In airborne communication environments, such as aircraft or satellite based systems, radio signal transmission/reception capability over a substantial terrestrial area is required. For example, in a satellite, the extent of terrestrial coverage is of shaped conical configuration substantially bounded by lines tangential to the surface of the earth and intersecting the satellite. For lower altitude aircraft radio coverage extends hemispherically from the aircraft to the horizon. Antennas located near the surface of the earth which communicate with high flying aircraft or satellites of undetermined location also require hemispherical coverage. In any of these environments, a requirement for intended hemispherical radio coverage is a signal transmission scheme that makes available more signal at elevation angles near the horizon because of the greater distance and transmission loss. In addition, and it is especially true for antennas mounted on high performance aircraft, the physical size and shape of the antenna impact directly on its utility in the environment. Ideally, the antenna should not only provide full hemispheric coverage with the desired increase in gain at near horizon elevation angles, but should also be rugged, light weight and be of low drag configuration, and thereby readily acceptable for mounting on high performance aircraft.
Prior art approaches to provide hemispherical antenna coverage have included turnstile and crossed-slot structures, as well as a combination of those two configurations, as exemplified by the multielement structure detailed in the U.S. patent to Griffee, et al., No. 3,811,127. As described in this patent, while a crossed-slot antenna presents a minimum height profile when mounted to the fuselage of the aircraft, in order to be satisfactorily broadband, it becomes too large in horizontal displacement for fuselage mounting. The turnstile approach suffers from maximum vertical height limitations, thereby making it too large for satisfactory mounting on modern jet aircraft.
The patentees' approach is to combine the turnstile and crossed-slot configuration in an effort to achieve broadband operation and still make the size of the antenna compatible with aircraft mounting limitations. However, the Griffee, et al. configuration must still be fairly large in order to obtain the broadband performance intended and the patentees do not contemplate adjustability or control of the shape of the radiation pattern.
Of course, reduced-size antenna structures, per se, such as those of microstrip configuration, have been proposed for airborne applications. Examples of such antennas are described in the U.S. patents to Kaloi, Nos. 4,125,838 and 4,151,530 and the U.S. patent to Van Atta, et al., No. 3,680,142. However, none of these structures provides a broad antenna pattern required for hemispherical coverage; nor do they provide control over the radiation pattern shape, in particular the ratio of zenith-to-horizon signal.